Liquid dairy products, such as milk, may be thermally processed to increase their stability. Unfortunately, thermally treating milk often results in color changes and/or gelation during processing or extended storage. For example, lactose in milk heated to high temperatures tends to interact with proteins and results in an unsightly brown color. This undesired condition is often referred to as browning or a browning or Mallaird reaction. Gelation, on the other hand, is not completely understood, but the literature suggests that gels may form, under certain conditions, as a three-dimensional protein matrix formed by the whey and casein proteins. See, e.g., Datta et al., “Age Gelation of UHT Milk—A Review,” Trans. IChemE, Vol. 79, Part C, 197-210 (2001). Both gelation and browning are undesirable in milk since they impart objectionable organoleptic properties.
The concentration of milk is often desired because it allows for smaller quantities to be stored and transported, thereby resulting in decreased storage and shipping costs, and may allow for the packaging and use of milk in more efficient ways. However, the production of an organoleptically-pleasing, highly concentrated milk can be difficult, because the concentration of milk generates even more pronounced problems with gelation and browning. For instance, milk that has been concentrated at least three-fold (3×) has an even greater tendency to undergo protein gelation and browning during its thermal processing. Additionally, such concentrated milk also has a greater tendency to separate and form gels over time as the product ages, thereby limiting the usable shelf life of the product. Concentrated milk, as a result, is generally limited to concentrations below about 25 percent total solids, protein levels below about 7 percent, and a shelf life of less than six months.
A typical method of producing concentrated milk involves multiple heating steps in combination with the concentration of the milk. For example, one general method used to produce concentrated milk involves first standardizing the milk to a desired ratio of solids to fat and then forewarming the milk to reduce the risk of the milk casein from coagulating during later sterilization. In that method, forewarming also decreases the risk of coagulation taking place during storage prior to sterilization and may further decrease the initial microbial load. The forewarmed milk is then concentrated by evaporation, ultrafiltration, or other appropriate methods to the desired concentration. The milk may be homogenized, cooled, restandardized, and packaged. In addition, a stabilizer salt may be added to help reduce the risk of coagulation of the milk that may occur at high temperatures or during storage. Either before or after packaging, the product is sterilized. Sterilization usually involves either relatively low temperatures for relatively long periods of time (e.g., about 90 to about 120° C. for about 5 to about 30 minutes) or relatively high temperatures for relatively short periods of time (e.g., about 135° C. or higher for a few seconds).
Various approaches for the production of concentrated milk have been documented. For example, Wilcox, U.S. Pat. No. 2,860,057, discloses a method to produce a concentrated milk using forewarming, pasteurizing, and high-temperature, short-term sterilization after concentration. Wilcox teaches the concentration of milk to approximately 26 percent solids using forewarming at about 115° C. (240° F.) for about 2 minutes prior to concentration, preheating at 93° C. (200° F.) for about 5 minutes after concentration, and sterilization at about 127 to 132° C. (261 to 270° F.) for 1 to 3 minutes.
U.S. Patent Publication US 2003/0054079 A1 (Mar. 20, 2003) to Reaves discloses a method of producing an ultra-high temperature milk concentrate having 30 to 45 percent nonfat milk solids. Reaves discloses the preheating of milk for 10 minutes at 65° C. (150° F.) to produce a preheated, milk starting product, which is then pasteurized at 82° C. (180° F.) for 16 to 22 seconds and evaporated under elevated pasteurizing temperatures (i.e., 10 minutes at 62° C. (145° F.) under vacuum) to produce an intermediate, condensed liquid milk. A cream and stabilizer, such as sodium hexametaphosphate or carrageenan, are added to the intermediate milk, which is then ultrapasteurized in two stages wherein the first stage is at 82° C. (180° F.) for 30 to 36 seconds and second stage is at 143° C. (290° F.) for 4 seconds. Shelf lives of 30 days to six months are reported for the resulting milk concentrate.
U.S. Patent Publication US 2007/0172548 A1 (Jul. 26, 2007) to Cale discloses a method to produce a heat-stable concentrated milk product by first forewarming a dairy liquid at a temperature of about at least about 60° C. for a time sufficient to effect a reduction of pH 4.6 soluble proteins. At low forewarming temperatures at about 60° C., Cale describes forewarming times of several hours. At higher temperatures, Cale describes lower times. For example, forewarming between about 70° C. and about 100° C. is described as requiring about 0.5 to about 20 minutes. Further concentration is then carried out by ultrafiltration either with or without diafiltration resulting in an intermediate dairy liquid having at least about 8.5 percent total protein. Stabilizers and mouthfeel enhancers are then added to the intermediate dairy liquid prior to sterilization. This composition is resistant to gelation and browning during sterilization and resistant to gelation and browning for at least about six months under ambient conditions.
As described in Cale, it is commonly believed that forewarming is a necessary process step in order to achieve the extended shelf-life of dairy concentrates. Cale describes that when an untreated dairy product (having both casein and whey proteins) is exposed to heat treatment, such as forewarming, it is believed that the whey proteins crosslink with the casein proteins (i.e., the κ-casein) present on the outer surface of casein micelles in the milk. Cale explains that such crosslinking accomplishes at least two effects. First, the interaction removes many of the whey proteins from solution, which is described as being important because whey protein can be very reactive at high temperatures, such as those experienced in sterilization. Secondly, as the casein micelles become coated with the serum or whey proteins, casein-casein interactions can be reduced or minimized, which is described as likely reducing the tendency of thermally induced milk gels to form.
Other types of concentration techniques for milk are known, but each technique generally has limited success in achieving shelf-stable or organoleptic pleasing concentrates. For example, Tziboula et al., “Microfiltration of milk with ceramic membranes: Influence on casein composition and heat stability,” Milchwissenschaft, 53(1):8-11, 1998 describes materials and methods directed to microfiltration of skim milk. Tziboula describes that the microfiltration of milk produces a retentate having both casein and whey, but also indicates that the whey in the retentate and permeate are in the same amounts. Tziboula concludes, however, the resultant retentates are generally less resistant to heat treatment as compared to the permeate or the starting milk source. Tziboula makes no provision for removing lactose from the retentates.